I was looking for something similar and found Brent Jackson's Colorable (Demo) http://jxnblk.com/colorable/demos/text/ . Sadly it does not allow you to enlarge the fonts and it is not based on the WCAG criteria.

I am wanting to develop a web page that goes through the different colour foreground and background for the same piece of text. To demonstrate colour contrast in a training session. Does anything like this exist already?